We have set our goals on, first, developing a 25 kDa sexual stage antigen, Pfs25, as a potential vaccine candidate; second, cloning the genes for the other known target antigens of transmission blocking immunity; third, identifying new target antigens on sexual stage parasites; and finally, understanding the molecular mechanisms involved in fertilization of malarial parasites. We have also begun to study the role of parasite glucose-6-phosphate dehydrogenase (G6PD) and its role in the putative protection afforded by G6PD deficiency in humans. Previously we had cloned the gene encoding Pfs25, a prime candidate antigen for a transmission blocking vaccine. Pfs25 has now been expressed in bacteria, yeast, vaccinia-infected mammalian cells and transiently transfected COS cells. Data from mice immunized with vaccinia-produced Pfs25 are very encouraging: sera from mice either inoculated with live, recombinant vaccinia or immunized with membrane extracts of mammalian cells infected with recombinant vaccinia block transmission of malaria. To begin further developing Pfs25 as a vaccine candidate, we have pursued engaging a number of commercial biotechnology companies in Cooperative Research and Development Agreements (CRADA). Cloning the other target antigens has been a problem. New approaches, such as mammalian expression systems, are being explored in addition to pursuing strategies previously successful, such as protein microsequencing and screening bacterial expression libraries. A microgamete (male) specific monoclonal antibody has been developed and may provide the entre we need to understand the molecular mechanisms involved in fertilization in the malaria parasite. Finally, we have demonstrated that the malaria parasite expresses G6PD constitutively, and independently of the G6PD status of the host. In our attempt to clone the parasite G6PD gene by complementation in E. coli, we have cloned the gene encoding a related enzyme, glucose phosphate isomerase (PGI).